Comparison

Electromagnetic vs Ultrasonic Flowmeters

Magnetic meters need conductive liquids. Ultrasonic meters can clamp on. Compare conductivity, installation, accuracy and maintenance.

Controls & Instrumentation · Last reviewed October 17, 2026

Short answer: choose an electromagnetic flowmeter for conductive liquids, including dirty water and slurries, when you can install it in the pipe and want high accuracy. Choose an ultrasonic meter when the fluid is not conductive, or when you cannot cut the pipe and a clamp-on meter will do.

	Electromagnetic (magmeter)	Ultrasonic (transit-time)
Principle	Voltage induced in a conductive liquid moving through a magnetic field	Difference in the travel time of sound pulses with and against the flow
Fluid requirements	The liquid must be conductive (often at least about 5 µS/cm). Not for oil, many solvents or pure gases	Works on many liquids and gases that carry sound. Needs clean fluid for transit-time. Doppler types use particles or bubbles
Pipe modification	In-line, with flanges. The pipe must be cut	Clamp-on needs none. Insertion and in-line types need a cut
Accuracy	High, often within 0.2 to 0.5% of the reading for good installations	Typically about 0.5 to 2%, depending on type, installation and the condition of the pipe
Full pipe	Needs a full pipe at the measuring point	Transit-time needs a full pipe, and some designs handle partial fill
Straight run	Short upstream and downstream straight runs	More sensitive to the flow profile, so straight run matters
Obstruction and pressure drop	No obstruction, so almost no pressure drop	None for clamp-on, minimal for the others
Maintenance	Electrodes and liner can foul or wear. Few moving parts	Clamp-on needs good acoustic coupling and clean pipe surface. Fouling and scale can affect the signal
Cost	Moderate to high, rising with size	Clamp-on is low in large sizes, and good for temporary or retrofit work

How to choose

Check the fluid: its conductivity, cleanliness, temperature and any gas content.
Check whether the pipe can be cut and how long the line can be down.
Specify the accuracy needed and the application: billing, process control or monitoring.
Check the available straight run and the pipe condition (material, lining, scale).
Consider long-term maintenance, including cleaning and calibration checks.

Common mistakes

Using a magmeter on deionized water, oil or a hydrocarbon, which are not conductive enough.
Mounting a meter where the pipe may not be full.
Using a clamp-on meter on a pipe with thick scale or a poor liner and trusting the accuracy.
Ignoring the flow profile effects of valves and elbows right upstream.

Related tools and labs

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Questions

Does a magmeter need straight pipe?

Less than many other meters, but the maker gives minimum upstream and downstream lengths. Follow them.

Can an ultrasonic meter measure gas?

Yes, with the right type. Gas meters are a separate design from the clamp-on liquid meters.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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